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1.
Deep magnetotelluric (MT) sounding data were collected and processed in the western part of the East European Craton (EEC). The MT sounding results correspond well with impedances obtained by magnetovariation (MV) sounding on the new geophysical observatory situated not far from the western border of Russia. Inversion based on combined data of both induction soundings let us evaluate geoelectrical structure of the Earth’s crust and upper and mid-mantle at depths up to 2000 km, taking into account the harmonics of 11-year variations. Results obtained by different authors and methods are compared with similar investigations on the EEC such as international projects CEMES in Central Europe and BEAR in Fennoscandia.  相似文献   

2.
The results of magnetovariational (MV) soundings are usually presented in the form of induction arrows. However, many examples show that the horizontal magnetic tensor (HMT) is more informative. The distribution of some HMT invariants directly traces the location of well-conducting rocks in the crust and upper mantle. The HMT determination requires simultaneous observations in an entire region, which is a substantial disadvantage. Yet, it is possible to apply techniques capable of restoring all the magnetic field components necessary for HMT estimation from tipper data arrays alone. These techniques exploit the spatial relationships between electromagnetic field components in a non-conducting atmosphere. For Central Europe, a large data set of induction arrows has been collected by the effort of many groups during the last 50 years. Based on these data, HMT values were calculated, and the results are very significant. The spatial behavior of certain HMT invariants demonstrates the presence of deeply seated, well-conducting rocks in the crust. Anomaly maximums display an arc-shaped trend that may be genetically linked with the Caledonian and/or Variscan margin thrust belts, which developed following the collision between Baltica and Avalonia and/or Gondwana-derived terranes, respectively. This is an important finding because the position of these deformation fronts in relation to the edge of the East European Platform is still controversial.  相似文献   

3.
We present the results of studying the geoelectrical structure of the zone of continental subduction of the Indian lithospheric plate within the Gahrwal Himalaya. In the framework of the Russian–Indian project, the data of the broadband magnetotelluric soundings conducted by the Indian Institute of Technology Roorkee on the regional profile across the structures of the orogen were expanded, processed, and interpreted by the new program tools adapted for the measurements in the mountain conditions and for the presence of industrial noise. The constructed model of the deep electrical conductivity cross section for Garhwal revealed its two-dimensional (2D) features and more accurately delineated the location of the midcrustal conductor associated with the ramp structure of the detachment plane. The correlations with the regional distribution of the earthquake hypocenters and the seismotomographic images suggest a common, fluid-related nature of the seismic and geoelectrical anomalies in the crust of the Garhwal Tectonic Corridor and enabled the identification of the seismogenerating zones. Among the data of the expanded profile set of magnetotelluric and magnetovariational transfer functions, the response of a poorly explored deep conductive body is revealed. This object is located east of the profile and is probably associated with the activation of the ancient trans-Himalayan cratonic structures which prepares the segmentation of the Himalayan arc.  相似文献   

4.
The hourly data of nine geomagnetic observatories situated in Central Europe have been analyzed using the generalized magnetovariation (GMV) method designed recently for induction soundings of inhomogeneous media. In this method, impedance is one of transfer functions in the differential relation between spectra of the magnetic components and their derivatives. The peculiarity of this impedance is its correspondence to the magnetotelluric one estimated from the linear relations. Three transfer functions have been estimated simultaneously for data of geomagnetic observatories, using three different routines working in the period range from three hours up to two days. Noises in the source field components have been compared with noise in the estimated plane field divergence. The multivariate errors-in-variables method was used to extract spatially and temporally coherent geomagnetic field structure from the partially incoherent geomagnetic variations. This method allows estimating reliably impedances and gradient tippers for each observatory, taking into consideration the Earth’s sphericity. The obtained responses have been used for induction soundings and for detecting a deep inhomogeneity in the region.  相似文献   

5.
An important result of recent years is the normal resistivity profile. It was obtained by interpretation of the combined apparent resistivity curve (magnetotelluric sounding and geomagnetic deep sounding) for the East European platform. This profile has no highly conducting layer and resistivity is greater than 100 ohm-m at asthenospheric depths. It corresponds well with geothermal indications of the absence of partial melting beneath the Precambrian plates. Nearly the same profiles have been obtained for the Canadian shield, and the Siberian and Australian platforms. Investigations carried out in many “hot” regions confirm the existence of a well-developed low-resistivity asthenosphere. Partially molten zones have conductances of about several thousand Siemens in the Eastern Pacific, Iceland and in the North American rift zone. Magnetotelluric soundings show that in many continental areas the lower part of the crust has low resistivity, in the range 10–20 ohm-m. Usually this crustal conductive layer is observed in regions of recent activity. Its total conductivity changes from several hundred to several thousand S. Many investigators propose that the most natural explanation of this conductivity is water solutions.It is necessary to note the distorting role of near-surface inhomogeneities. Local distortions can be eliminated by simple averaging of the experimental data. These average apparent resistivity curves are the starting point for the construction of deep geoelectrical models.  相似文献   

6.
Jamal Asfahani 《水文研究》2007,21(21):2934-2943
Twenty‐nine Schlumberger electrical soundings were carried out in the Salamiyeh region in Syria using a maximum current electrode separation of 1 km. Three soundings were made at existing boreholes for comparison. Aquifer parameters of hydraulic conductivity and transmissivity were obtained by analysing pumping test data from the existing boreholes. An empirical relationship between hydraulic conductivity determined from the pumping test and both resistivity and thickness of the Neogene aquifer has been established for these boreholes in order to calculate the geophysical hydraulic conductivity. A close agreement has been obtained between the computed hydraulic conductivity and that determined from the pumping test. The relationship established has, therefore, been generalized in the study area in order to evaluate hydraulic conductivity and transmissivity at all the points where geoelectrical measurements have been carried out. This generalization allows one to derive maps of the hydraulic conductivity and transmissivity in the study area based on geoelectrical measurements. These maps are important in future modelling processes oriented towards better exploitation of the aquifers. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

7.
One simulation and two field examples from New Jersey illustrate resolution improvement in geoelectrical soundings applied to groundwater exploration. Layered-earth parameter resolution is derived from data obtained with the commonly used methods of resistivity, induced polarization (IP) and transient electromagnetic (TEM) soundings. Resolution improvement is achieved by simultaneous inversion of two or more data sets and by constraining parameters of the inverse problem. A quantitative analysis showing the contribution of IP data to the resolution of geo-electric sections is presented. Simultaneous inversion of simple IP data with conventional resistivity and resistivity-TEM data sets resulted in improved parameter resolution. IP data improved resolution in three ways: (1) by decoupling correlated layered-earth parameters, (2) by adding information to a geological interpretation about a second physical property, namely chargeability, and (3) by increasing the electrical information available.  相似文献   

8.
A method has been developed for computation of the electrical DC potential of an arbitrary three-dimensional resistivity structure using a finite difference method. The threedimensionality is necessary for interpretation of geoelectrical soundings with controlled point sources over a laterally inhomogeneous medium. Lateral inhomogeneities should be considered in resistivity soundings with large layouts. The results obtained with the described method permit a more realistic representation of geological features. The resolution of the method is determined by the number of elements in the resistivity network. The problem of core memory space has been resolved by using random access disc files. The results computed using a Fortran program are in good agreement with analytically obtained solutions.  相似文献   

9.
This study is devoted to the development of a method for deep tensor electromagnetic inductive sounding of the Baltic Shield lithosphere with the use of mutually orthogonal industrial power lines alternately connected to a generator of up to 100 kW in power. The first results of the sounding are obtained at separations of up to 510 km from the source. The electrical conductivity of the Baltic Shield lithosphere in the crust-mantle transition zone (depths of 20–50 km) is numerically estimated from soundings with natural and controlled sources. The perspectives of future investigations are associated with synchronous observations on a 2-D network of measuring stations. This will enable the study of the relation of deep geoelectric parameters of the lithosphere to the geological structure of the crust in terms of 3-D models.  相似文献   

10.
An audio-magnetotelluric investigation in Terceira Island (Azores)   总被引：2，自引：0，他引：2  
Ten audio-magnetotelluric soundings have been carried out along a profile crossing the Serra do Cume caldera in the eastern part of the Terceira Island (Azores). The main objectives of this investigation were to detect geoelectrical features related with tectonic structures and to characterize regional hydrological and hydrothermal aspects mainly those related to geothermal fluid dynamics.Three-dimensional numerical investigation showed that the data acquired at periods shorter than 1 s are not significantly affected by ocean effect. The data was analysed using the Smith's decomposition method in order to investigate possible distortions caused by superficial structures and to estimate a global regional strike. The results suggest that in general the soundings were not distorted. A regional N55°W strike was chosen for the two-dimensional data inversion.The low-resistivity zones (10–30 ohm-m) displayed in the central part of the 2-D geoelectrical model have been interpreted as caused by hydrothermal circulation. The low-resistivity anomalies at the ends of the profile might be attributed to alteration zones with interaction of seawater intrusion. High-resistivity (> 300 ohm-m) values have been related with less permeable zones in the SW of Cinco Picos and Guilherme Moniz caldera walls.  相似文献   

11.
A series of geomagnetic induction studies has been undertaken in Scotland since 1973. It includes the operation of two geomagnetic arrays, one over northern Scotland and the other over southern Scotland, subsequent individual station and small-scale geomagnetic array studies, and three sets of magnetotelluric soundings which traverse Scotland from the Lewisian Foreland to the English border. The problems associated with the interpretation of induction data from an island located in the subauroral region are discussed qualitatively and the manner in which both coast and source field effects can be accounted for, is described. The geomagnetic deep sounding data (GDS) from all the observation sites have been collated, and examples of hypothetical event contour maps and traverses across the Great Glen and of individual events from the northern array are presented. They indicate that significant lateral variations in electrical conductivity structure within the crust and upper mantle are associated with the major geological faults in the region. Examples of the results of the magnetotelluric soundings are also presented, together with an outline of the procedure used for one- and two-dimensional modelling. Models of the geo-electric structure in both northern and southern Scotland have been obtained. These show distinctive features which are compatible with the qualitative interpretation of the magnetovariational data. For example, the major granitic blocks are highly resistive while regions of relatively low resistivity exist at upper crustal depths near the Great Glen, Highland boundary and Southern Uplands faults. A zone of low resistivity exists at lower crust-uppermost mantle depths throughout much of the region, the lowest value occurring under the Southern Uplands.  相似文献   

12.
The geological-geophysical and petrological-geochemical studies of the Earth’s crust and upper mantle are combined to estimate the state of the lithosphere at the junction zone of Tarim and Tien Shan. The laboratory measurements of electric conductivity in the rocks sampled from the upper mantle and lower crust considered against the geoelectrical and thermal models revealed lherzolite, granulite, and eclogite massifs in the deep section of the Tarim and Tien Shan junction zone. The experimental results suggest that the crustal thickness in the southern Tien Shan attained 35–40 km 70 Ma ago.  相似文献   

13.
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14.
On the basis of exploration magnetotelluric soundings (MTS) carried out by the Tyumen Geological Survey in 1980–1981 within the Tobol-Ishim interfluve, the top of the conductive horizon with the electric resistivity of dozens of Ω m is identified in the geoelectrical section at a depth of 75–80 km. Two segments of the locally elevated top of this horizon are revealed. One segment, where the depth of the top of the conductor is 60–65 km, corresponds to the region of the Kiselevskii fault-the main fault in this territory; another segment is associated with the edge zone of the Ishim branch of the Triassic rift system within the West Siberian Plate. Within this segment, the top of the conductive layer rises to a depth of 55-60 km. It is assumed that the Ishim geoelectrical anomaly, which projects into the geothermal anomaly and is coherent with the features of the deep geological structure of its display region, is conditioned by the element of the mantle-crustal magmatogene fluid paleosystem.  相似文献   

15.
The results of the magnetotelluric (MT) soundings before and after an earthquake are analyzed. The interpretation is based on the longitudinal and transverse MT curves (along and across the strike of the main tectonic elements, respectively). The MT curves are distorted by the ρ- and coast effect. The distortions due to the coast effect are estimated by the testing three-dimensional (3D) model. It is established that the coast effect distortion at the periods up to 1000 s is small and can be disregarded. The divergence of the longitudinal and transverse MT curves, which points to the presence of the deep faults, is thoroughly studied. The inversion of the MTsounding curves is carried out by the REBOCC program of the numerical two-dimensional modeling. This program implements the procedures of elimination of the ρ-effect and the joint inversion of the longitudinal and transverse MT sounding curves. The obtained geoelectrical cross sections provide an insight into the structure of electrical conductivity of the lithosphere before and after the earthquake. The more intense variations in the electric conductivity are observed in the zone of the deep faults. These variations are related to the changes in the porosity and saturation of the rocks by the highly mineralized fluids.  相似文献   

16.
A joint inversion of gravity and geoelectrical data has been performed along a 100 km long profile across the oil-bearing Potiguar rift basin in NE Brazil. The integrated approach is based on the connection between density and resistivity of a sedimentary sequence through the porosity. Seventy-one gravity stations and twenty-nine vertical electrical soundings were carried out across the central part of the Early Cretaceous basin in the Precambrian Borborema Province to apply the proposed joint inversion. Both the physical properties and geometry of the multilayer deep model were well constrained by a wide set of prior information obtained by surface mapping, geophysical logs and seismic sections. The results reveal a rift architecture formed by a 5000 m thick half-graben structure separated by an intervening basement high and an extensive adjacent platform with a sedimentary infill of about 300 m thick. The calculated model shows geometries for the sedimentary density/resistivity interfaces in agreement with the seismostratigraphic sequences revealed by seismic section, representing a substantial improvement in comparison with previously published gravity models.  相似文献   

17.
The problem of interpretation of geoelectrical resistivity soundings has been studied and a compromise method of interpretation developed. A simple depth sequence of increasing bed thickness is taken and resistivities assigned to the various layers so that a good fit is obtained to the observed apparent resistivity curve. An initial estimate of the resistivities is made by a direct method but the model is then altered to give an improved fit by the indirect method of curve matching. The whole method is computerized and input data to the computer program consists of spacings and apparent resistivities. The method is extremely rapid, about one third of the computation time is devoted to obtaining the resistivity transform by the method of convolution. A normal type of solution, incorporating additional information if necessary, can also be obtained.  相似文献   

18.
Magnetotelluric sounding data obtalned recently in Manas earthquake area were processed. Inthe result, curves of apparent resistivity, impedance Phase, skewness and optimum rotationangle versus period and the real magnetic induction vectors were obtained. Then the data ofall measuring points were interpreted by 2D automatic inversion. The result indicates thatalong the sounding profile the shallow crust can be divided into 5 segments and the deep crustcan be divided into 3 segments, with faults or deep-seated fault zones as the contactboundaries between them. The sedimentary cover along the profile extents down to depthabout 12 km in maximum and a low-resistivity body exists in the crust in southern section ofthe profile. The interpretation results are well consistent with geological and othergeophysical data. The Manas M7. 7 earthquake occurred near a contact zone where theelectrical structure of the crust sharply changes.  相似文献   

19.
Magnetotelluric and differential geomagnetic sounding surveys, consisting of nine soundings, were performed in 1984 along a 200-km profile across the southern Senegal basin. They were intended to obtain information concerning the resistivity structure of the crust and upper mantle and the distribution of the induced electric currents. Magnetotelluric data indicate that two-dimensional resistivity models are appropriate for the region. The zone above the basement is highly inhomogeneous in geoelectrical structure. Very conductive sediments (0.6-3 ohm m) appear in the Mesozoic-Cenozoic Senegal basin. These sediments lie at depths of up to 4500 m on the west end of the profile. Below this, a modest resistivity material (10–30 ohm m) extends to a maximum depth of about 3000 m. The material at depth on the cast part of the traverse line is thought to be Palaeozoic sediments of the Bove basin. The depth of the magnetotelluric basement lies between about 250 m (in the east) and 4800 m (in the west). The crust is characterised by a drop in electrical resistivity at a depth of 15 km below the east part of the profile. Considering the total section, we observe a general trend towards lower resistivities at depths in excess of 100 km, the transition from 2000 ohm m to about 2 ohm m occurs in the depth range 100 to 175 km. An analysis of the geomagnetic variation field has identified a concentration of telluric current flow beneath the deep basin. It appears that the additional currents flowing in the striking direction of the Senegal basin are largely controlled by sedimentary rocks of high conductivity lying at depths less than 5 km. Model studies show that the local conductivity distribution is able to explain the currents circulating in the thick well-conducting sediments.  相似文献   

20.
Summary Phase velocities of Rayleigh waves propagating along the Prague-Warsaw profile have been determined. A layered model of the Earth's crust and upper mantle, satisfying the observed dispersion of Rayleigh waves, has been derived. In constructing this model, the results of deep seismic soundings along international profile VII were also taken into account.  相似文献   

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